src/Normalizer/NormalizerBase.php | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/src/Normalizer/NormalizerBase.php b/src/Normalizer/NormalizerBase.php
index 14b1a5a..709739c 100644
--- a/src/Normalizer/NormalizerBase.php
+++ b/src/Normalizer/NormalizerBase.php
@@ -16,4 +16,15 @@ abstract class NormalizerBase extends SerializationNormalizerBase {
    */
   protected $format = 'api_json';
 
+  /**
+   * {@inheritdoc}
+   */
+  protected function checkFormat($format = NULL) {
+    // The parent implementation allows format-specific normalizers to be used
+    // for formatless normalization. The JSON:API module wants to be cautious.
+    // Hence it only allows its normalizers to be used for the JSON:API format,
+    // to avoid JSON:API-specific normalizations showing up in the REST API.
+    return $format === $this->format;
+  }
+
 }